The goal of the project is to determine what percentage of heart muscle cells is capable of DNA synthesis and division at different developmental stages and thus to be able to estimate what amount of functional regeneration and repair can be expected after congenital or surgical damage to the heart during development and growth. Myocardial cells from chick embryos, post-hatching chicks and human fetal hearts of different developmental ages are grown as monolayer cell cultures or tissue cultures. Proliferating cells are labeled as they enter the DNA synthesis portion of the cell cycle by H3 thymidine continuously present in the growth medium. Cultures and tissue pieces are fixed or glycerine extracted after different periods of exposure to H3-thymidine, and either embedded and sectioned for electron microscopy or treated with fluorescein conjugated antibody against the muscle protein myosin. Both preparations are coated with photographic emulsion for autoradiography, and cells are scored for H3-thymidine-labeled nuclei and fluorescent myofibrils by fluorescence microscopy or thick (myosin) filaments by electron microscopy. From this data the proliferative fraction of heart muscle cells at each developmental age can be calculated. By plotting proliferative fraction verses age, we can determine at what point in development heart muscle cells become a non-dividing population. Similar experiments on chick hearts labeled in vivo with H3-thymidine injections will determine the extent to which myocardial cell proliferation in vivo differs from that in vitro.